ANN ARBOR, Mich.—Freshwater from melting ice sheets set the stage several
thousand years ago for production of natural gas along the margins of sedimentary
basins.

Now researchers at the University of Michigan and Amherst College are reading
chemical signatures of water in those areas to pinpoint places where gas is
most likely to be found. Their most recent work is described in a paper published
in the May/June issue of the Geological Society of America Bulletin.

Natural gas forms when organic materials trapped in sediments decompose. This
can happen when the materials are exposed to high temperatures, producing thermogenic
gas, or when bacteria break down the organic matter and, through the process
of methanogenesis, produce microbial gas.

Finding and exploiting microbial gas deposits, which account for as much as
20 percent of the world's natural gas resources, is "becoming more and
more important," said U-M doctoral student Jennifer McIntosh, lead author
of the paper. "And if you're exploring for microbial gas, you need to
know what areas have been affected by methanogenesis, because that's how the
microbial gas is produced."

McIntosh and coauthors Lynn Walter, U-M professor of geological sciences,
and Anna Martini, assistant professor of geology at Amherst College, studied
Antrim Shale deposits in the Michigan Basin, a deep depression filled with
sediments that date back to the Paleozoic Era. While thermogenic gas forms
far below the surface in the centers of such depressions, microbial gas is
produced along the shallow edges. In previous work, the researchers showed
that freshwater seeping into basin edges from melting ice sheets made it possible
for methanogenesis to occur. "The fluids in the Michigan basin are some
of the most saline fluids in the world," McIntosh said. "When freshwater
penetrated into these basin margins, it suppressed the salinity and created
an environment that was conducive to methanogenesis within organic-rich black
shales."

In the current work, the research team compared the chemistry of water from
wells drilled in the deeper center of the basin with that of water from wells
at the edges. Their analysis not only provided further evidence that melting
ice sheets made it possible for methane-producing bacteria to inhabit the shallow
deposits, but also showed that methanogenesis has significantly changed water
chemistry in those areas.

"We see large decreases in the calcium-to-magnesium and calcium-to-strontium
ratios in high bicarbonate waters associated with microbial gas deposits,
indicating methanogenesis caused calcite to precipitate within the Antrim
Shale," McIntosh said. "So you can use the elemental chemistry
of these shale wells to be able to tell if there was methanogenesis, and
that guides gas companies in terms of where to explore for microbial gas.
It's a relatively inexpensive analytical tool, compared to other methods
that have been used, such as stable isotope chemistry."

The method has potential not just in Michigan, but also in the Illinois basin
and in other parts of the world that have similar black shale deposits, said
McIntosh. "There are organic-rich deposits in many basins throughout the
world, and a lot of these have been covered by continental ice sheets, so these
may represent areas where freshwater has penetrated into basins and microbial
gas has been generated." To explore that idea, McIntosh compiled water
chemistry data from basins in Africa, Asia and North America. "I was able
to see similar trends in the water chemistry in other areas with microbial
gas deposits, showing how important microbial processes may be in changing
the fluid chemistry within the earth's crust," she said.

The research was funded in part by the Petroleum Research Fund, administered
by the American Chemical Society, and by the Gas Research Institute.